Sabine Strahl

Protein O-Mannosylation and its diverse interplay with N-Glycosylation

Protein O-mannosylation and N-glycosylation are fundamental processes that are essential for the growth and development of eukaryotes. Both protein modifications initiate at the endoplasmic reticulum. Properly glycosylated and folded proteins can journey on to the Golgi apparatus where N-linked and O-mannosyl glycans are further trimmed and/or elongated. Ultimately, glycoproteins with highly diverse glycans emerge. The importance of these protein modifications becomes obvious in light of the fact that defects in both types of glycosylation result in severe human diseases such as the congenital disorders of glycosylation (CDGs).

In the endoplasmic reticulum, the N-glycosylation machinery and protein O-mannosyltransferases compete for the sugar donor dolichol phosphate-activated mannose. There is now growing evidence that the connection between O-mannosylation and N-glycosylation extends far beyond their sharing of a common sugar donor (reviewed in Neubert and Strahl, 2016). Our recent work revealed that the inhibition of mammalian protein O-mannosyltransferases negatively affects E-cadherin-mediated cell-cell adhesion, via unexpected crosstalk between O-mannosylation and the biosynthesis of individual N-glycan structures on this important adhesion molecule (Lommel et al., 2013; Carvalho et al., 2016). It is still unclear, however, whether the observed effect is restricted to E-cadherin or whether N-glycosylation is generally affected by a decrease in O-mannosylation, and what mechanisms are behind this coordinated interplay.

To address these questions, we aim to study mammalian O-mannosylation and its relation to N-glycosylation at the operating, regulatory and functional levels. Our main objectives are i) to unravel the mode of action of the mammalian O-mannosyltransferases in the endoplasmic reticulum; and ii) to explain the consequences of defective O-mannosylation at the molecular level with a focus on cadherins.

In combination, our analyses will provide a comprehensive view of the initiation of O-mannosylation in the endoplasmic reticulum as well as the diverse implications of O-mannosylation defects, and guide the analyses of physio-pathologic abnormalities in CDG models.

Figure 1: The biosynthetic pathway of N-linked and O-mannosyl glycans in mammals.

In the ER, assembly of the mannose donor Dol-P-Man and the N-glycan precursor Glc3Man9GlcNAc2-PP-dolichol take place. At the translocon complex, the oligosaccharide is transferred by the oligosaccharyltransferase complex (OST) to Asn present in the consensus sequence N-x-S/T of acceptor proteins. Further trimming steps of the N-glycan in the ER are not illustrated. O-Man is initiated by POMT complexes by the transfer of mannose from Dol-P-Man to Ser and Thr residues of acceptor proteins. Whether mammalian POMTs also act at the translocon is not known. In mammals, upon ER exit, the initial protein-linked glycans may be extended step-wise. Numerous different glycosyltransferases contribute to this process. During N-Glyco high mannose, hybrid and complex type glycans and their derivatives are assembled (left panel). During O-Man core M1, M2 and M3 glycans, and their derivatives are synthesized. In the absence of the β1,2-GlcNAc transferase POMGnT1 no extended core O-mannosyl glycans are formed (right panel). (gift from P. Neubert)

Team

Dr. Ricardo Bruno da Costa Pereira de Carvalho has earned his PhD at Heidelberg University studying centriolar satellites and control of ciliogenesis. He is working on the mode of action of mammalian POMT O-mannosyltransferases in the endoplasmic reticulum.

Dr. Sina Noor has earned his PhD at University of Kaiserslautern working on mammalian monocarboxylate transporter. He is involved in analyses addressing the consequences of defective O-mannosylation.